location, location, location
One L.A. area resident here [20 miles East of the city borders actually].
On the point of city physical size, Los Angeles was by far the largest city in the US during the 40's. It's size was a standing joke. Other cities now match and pass it, but it is still one of the largest.
While as noted, we are a definitely biased sample, and most likely in several ways not immediately apparent to us, but we may well be reasonably accurate in thinking there is a Midwest bias to the game. The Living Greyhawk campaign has several regions in that area, some that only cover a state. By contrast, the population of outer regions is much larger.